The electroluminescent phenomenon of organic materials was discovered by Pope et al. in 1963, when a 5 mm single layer anthracene crystal obtained by thermal evaporation was used as an emitting layer and the driving voltage for the organic light-emitting device thus produced must be as high as 100 V or above. In 1987, Dr. Ching W. Tang et al. of Eastman Kodak Company, USA, made a double layer device with an organic fluorescent dye by vacuum thermal evaporation, of which the driving voltage was smaller than 10 V. Currently, an OLED device is produced by lamination, in which an anode layer formed of a transparent conductive material such as indium tin oxide (ITO) is provided on a glass substrate, and on the anode layer, a hole transporting layer (HTL), an emitting layer (EML), a hole blocking layer (HBL), an electron transporting layer (ETL), an electron injection layer (EIL) and a cathode layer are provided in order.
The anode layer can be constituted of ITO, and the cathode layer can be constituted of metals having a low work function (such as Al, Mg or their alloys with other metals). The host or guest of the emitting layer can be constituted of metal complexes or common organic compounds. The hole blocking layer commonly uses organic compounds such as BCP (2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline). The material for the hole transporting layer was mainly triarylamines, such as TPD (N,N′-diphenyl-N,N′-bis(3-methylphenyl)-1,1′-biphenyl-4,4′-diamine), NPB (N,N′-diphenyl-N,N′-bis(1-napthyl)-(1,1′-biphenyl)-4,4′-diamine), etc., in the past. The material for the electron transporting layer commonly uses Alq3 (Tris(8-hydroxyquinolinato) aluminum). The material for the host of an emitting layer commonly uses CBP (4,4′-bis(carbazol-9-yl)biphenyl). The material for the guest of a red light emitting layer commonly uses rubrene ((5,6,11,12)-Tetra-phenylnaphthacene), and the material for the guest of a green light emitting layer commonly uses Ir(ppy)3 (Tris(2-phenylpyridine)iridium).
The organic light-emitting diode device is essentially composed of organic molecules, in which derivatives from quinoxaline that use it as their main structures have been used as the medium of organic electroluminescence. For example, the OLED device disclosed by Peter Strohriegl et al. uses a quinoxaline derivative as the material for the electron transporting layer (Macromolecules 1998, 31, 6434-6443), and the structure of its substituent includes Bis(phenylquinoxalines); the OLED device disclosed by Hans-Werner Schmidt et al. uses a quinoxaline derivative as the material for the electron transporting layer (Phys. Chem. chem. phys 1999, 1, 1777-1781), and the structure of its substituent includes spiroquinoxaline. The OLED device disclosed by EP 2065378 uses a quinoxaline derivative as the material for the electron transporting layer, and the structure of its substituent includes pyridyl, alkyl, aryl or arylene but does not include amino. The OLED device disclosed by U.S. Pat. No. 7,265,378 uses a quinoxaline derivative as the material for the electron transporting layer, and the structure of its substituent includes halogen, haloalkyl, aryl, etc., but does not include amino.
In the aforementioned OLED devices, those quinoxaline derivatives are mostly used as the material for the electron transporting layer, and it has not been disclosed that quinoxaline derivatives can be used as the materials for the hole transporting layer and the host or guest of the emitting layer simultaneously or solely. Generally speaking, the hole transporting layer, the electron transporting layer and the emitting layer are constituted of different main structural materials.